Generally, the LCD device has features of thin, light-weight, and low power consumption.
In particular, an active-matrix addressing LCD (AM-LCD) device that drives respective pixels arranged in a matrix array with active elements has ever been recognized as a high image quality flat panel display device. Especially, the AM-LCD device using thin-film transistors (TFTs) as the active elements is widely used as TFT-LCD devices.
Most of the TFT-LCD devices, which utilize the electro-optic effects of TN (Twisted Nematic) type liquid crystal sandwiched between two substrates, display images by applying an electric field approximately vertical to the surfaces of the substrates across the liquid crystal to thereby cause displacement of the liquid crystal (LC) molecules. These LCD devices are termed “vertical electric field type”.
On the other hand, some LCD devices display images by applying an electric field approximately parallel to the surfaces of the substrates to thereby cause displacement of the LC molecules in the planes parallel to the surfaces of the substrates. These LCD devices are termed “lateral electric field type” or “in-plane switching (IPS) mode”. Various improvements have ever been made for the IPS-mode LCD devices too. Some of the improvements will be exemplified below.
A structure using comb-tooth-like electrodes mated with each other in the IPS-mode LCD device is disclosed in U.S. Pat. No. 3,807,831 (patent document 1) issued in 1974 (refer to claim 1, FIGS. 1-4 and FIG. 11).
A technique using the comb-tooth-like electrodes mated with each other similar to those in the above-mentioned patent document 1 in the IPS-mode AM-LCD device utilizing the electro-optic effects of the TN type liquid crystal is disclosed in Japanese Unexamined Patent Publication No. 56-091277 (patent document 2) published in 1981 (refer to claim 2, FIG. 7 and FIGS. 9 to 13). This technique reduces the parasitic capacitance between a common electrode and drain bus lines, or that between the common electrode and gate bus lines.
A technique that realizes the IPS-mode LCD device without the comb-tooth-like electrodes in the AM-LCD device using TFTs is disclosed in Japanese Unexamined Patent Publication No. 7-036058 (patent document 3) published in 1995 (refer to claims 1 and 5, FIGS. 1 to 23). With this technique, the common electrode and image signal electrodes or the common electrode and LC driving electrodes are formed on different layers and at the same time, the common electrode or the LC driving electrodes is/are formed to be ring-shaped, cross-shaped, T-shaped, Π(Greek letter Pi)-shaped, H-shaped, or ladder-shaped.
A structure that the pixel electrode and the common electrode for generating the LC driving lateral electric field (both of which are comb-tooth-shaped) are disposed above (i.e., at closer positions to the LC layer) the bus lines (i.e., data lines) that supply signals to the active elements for driving respective pixels, where an insulating layer intervenes between the pixel electrodes and the common electrode, is disclosed in Japanese Unexamined Patent Publication No. 2002-323706 (patent document 4) published in 2002 (refer to claim 1, first exemplary embodiment, FIGS. 1 to 2). It is said that with this structure, since the electric field from the bus lines can be shielded by forming the common electrode to cover the bus lines, defective display caused by vertical crosstalk is prevented. Moreover, it is said that an aperture ratio is increased by forming the common electrode with transparent conductive material.
FIG. 11A through FIG. 11C are drawings explaining an example of the structure of a related-art popular IPS-mode AM-LCD device. FIG. 11A is a plan view of the device, FIG. 11B is a cross-sectional view along the I-I line shown in FIG. 11A, and FIG. 11C is a cross-sectional view along the II-II line shown in FIG. 11A. Moreover, FIG. 12A through FIG. 12D are partial plan views showing fabrication steps of the related-art LCD device. All of these drawings show the structure of one pixel region.
With the related-art LCD device, as shown in FIG. 11A and FIG. 12B, rectangular regions are formed by gate bus lines 55 extending along a horizontal direction of FIG. 11A and FIG. 12B and drain bus lines 56 extending along the vertical direction thereof. Pixel regions are formed in the respective rectangular regions. Pixels are arranged in a matrix array as a whole.
Common bus line 53 is formed to extend along the horizontal direction of FIG. 11A and FIG. 12B for each pixel, similar to the gate bus lines 55. At the respective intersections of the gate bus lines 55 and the drain bus lines 56, TFTs 45 (see FIG. 11A and FIG. 11B) are formed corresponding to the respective pixels. A drain electrode 41, a source electrode 42, and a semiconductor film 43 of each TFT 45 are formed to have patterns or shapes shown in FIG. 12B, respectively.
The pixel electrode 71 and the common electrode 72, which generate a liquid-crystal (LC) driving electric field, configure comb-tooth-like portions (i.e., thin belt-shaped parts protruding into the pixel region) mated or engaged with each other, respectively. Here, as an example, the number of the comb-tooth-like portions of the pixel electrode 71 is two and the number of the comb-tooth-like portions of the common electrode 72 is one.
As shown in FIG. 11B, the pixel electrode 71 is electrically connected to the corresponding source electrode 42 of the TFT 45 by way of a corresponding contact hole 61 that penetrates through an organic interlayer film 60 and a protective insulating film 59.
The common electrode 72 is electrically connected to the corresponding common bus line 53 by way of a corresponding contact hole 62 that penetrates through the organic interlayer film 60, the protective insulating film 59, and an interlayer insulating film 57.
Part of the source electrode 42 of the TFT 45 is overlapped with the corresponding common bus line 53, thereby forming a storage capacitor for the pixel region by the overlapped part.
The cross-sectional structure of the related-art LCD device is shown in FIG. 11B and FIG. 11C, where this device is configured by coupling and unifying an active-matrix substrate and an opposite substrate to sandwich a liquid crystal layer between them.
The active-matrix substrate comprises a transparent glass substrate 11, the common bus lines 53, the gate bus lines 55, the drain bus lines 56, the TFTs 45, the pixel electrode 71, and the common electrode 72, all of which are formed on or over an inner surface of the glass substrate 11. The common bus lines 53 and the gate bus lines 55, which are directly formed on the inner surface of the glass substrate 11, are covered with the interlayer insulating film 57. The drain electrodes 41, the source electrodes 42, and the semiconductor films 43 of the TFTs 45, and the drain bus lines 56 are formed on the interlayer insulating film 57. Thus, the common bus lines 53 and the gate bus lines 55 are electrically insulated from the drain electrodes 41, the source electrodes 42, the semiconductor films 43, and the drain bus lines 56 by the interlayer insulating film 57.
These structures formed on the glass substrate 11 are covered with the protective insulating film 59 except for the regions where the contact holes 61 and 62 are formed. The level differences caused by the contact holes 61 and 62 are planarized by the organic interlayer film 60 formed on the protective insulating film 59. The pixel electrode 71 and the common electrode 72 are formed on the organic interlayer film 60.
As explained above, the pixel electrode 71 is electrically connected to the corresponding source electrode 42 by way of the corresponding contact hole 61, and the common electrode 72 is electrically connected to the corresponding common bus line 53 by way of the corresponding contact hole 62. In addition, the cross-sectional views of FIG. 11B and FIG. 11C are schematically drawn and thus, they do not reproduce the actual level differences faithfully.
The surface of the active matrix substrate having the above-described structure, on which the pixel electrode 71 and the common electrode 72 are formed, is covered with an alignment film 31 formed by an organic polymer film. The surface of the alignment film 31 has been subjected to an alignment treatment for directing an initial orientation direction of LC molecules 21 to a desired direction (see a both way arrow 30 in FIG. 11A).
On the other hand, an opposite substrate (i.e., a color filter substrate) includes a transparent glass substrate 12; and a color filter (not shown) of three primary color layers of red (R), green (G) and blue (B) being formed so as to correspond to the respective pixel regions, and a light-shielding black matrix (not shown) formed on the regions other than those corresponding to the respective pixel regions. The color filter and the black matrix, which are formed on the inner surface of the glass substrate 12, are covered with an acrylic-based overcoat film (not shown).
On the inner surface of the overcoat film, columnar spacers (not shown) are formed to control a gap between the active-matrix substrate and the opposite substrate. The inner surface of the overcoat film is covered with an alignment film 32 formed by an organic polymer film. The surface of the alignment film 32 has been subjected to an alignment treatment for directing the initial orientation direction of the LC molecules 21 to a desired direction (see the both way arrow 30 in FIG. 11A).
The active-matrix substrate and the opposite substrate each having the above-described structure are overlapped on each other at a predetermined gap in such a way that their surfaces on which the alignment films 31 and 32 are respectively formed are directed inward and opposed to each other. Liquid crystal 20 is introduced into the gap between these two substrates. The peripheries of the substrates are sealed by a sealing member (not shown) to confine the liquid crystal 20 therein. A pair of polarizer plates (not shown) is arranged on the outer surfaces of the substrates, respectively.
The surfaces of the alignment films 31 and 32 have been uniformly alignment-treated in such a way that the LC molecules 21 are aligned in parallel along the desired direction when no electric field is applied, as described above. The alignment direction by the alignment treatments is a direction inclined clockwise by 15 degrees with respect to the direction along which the comb-tooth-like portions of the pixel and common electrodes 71 and 72 are extended (i.e., the vertical direction in FIG. 11A).
The transmission axes of the pair of polarizer plates are crossed at right angles. The transmission axis of one of the pair of polarizer plates is in accordance with the initial alignment direction of the LC molecules 21 determined by the uniform alignment treatment.
Next, the fabrication process steps of the related-art LCD device shown in FIG. 11A through FIG. 11C will be explained below with reference to FIG. 12A through FIG. 12D.
The active-matrix substrate is fabricated in the following way. First, a chromium (Cr) film is formed on one of the surfaces of the glass substrate 11 and patterned, thereby forming the common bus line 53 and the gate bus lines 55 having the shapes as shown in FIG. 12A. Thereafter, the interlayer insulating film 57, which is formed by a silicon nitride (SiNx) film, is formed to cover the common bus line 53 and the gate bus lines 55 over the whole surface of the glass substrate 11.
Subsequently, the semiconductor films 43 (which are usually formed by an amorphous silicon (a-Si) film) are formed to have island-shaped patterns on the interlayer insulating film 57 in such a way to be overlapped with the corresponding gate bus lines 55 by way of the interlayer insulating film 57.
Another Cr film is then formed on the interlayer insulating film 57 and patterned, thereby forming the drain bus lines 56, the drain electrodes 41 and the source electrodes 42 (see FIG. 12B). Thereafter, the protective insulating film 59 made of SiNx and the organic interlayer film 60 made of photosensitive acrylic resin are successively laminated on the interlayer insulating film 57 in this order to cover these structures.
Following this, the rectangular contact holes 61 penetrating through the protective insulating film 59 and the organic interlayer film 60 and the rectangular contact holes 62 penetrating through the interlayer insulating film 57, the protective insulating film 59, and the organic interlayer film 60 are formed (see FIG. 12C).
An TTO (Indium Tin Oxide) film, which is a transparent conductive material, is formed on the organic interlayer film 60 and patterned, and thereby forming the pixel electrode 71 and the common electrode 72 on the organic interlayer film 60. The pixel electrode 71 is in contact with the corresponding source electrode 42 by way of the corresponding contact hole 61. The common electrode 72 is in contact with the common bus line 53 by way of the corresponding contact hole 62 (see FIG. 12D and FIG. 11B). In this way, each pixel region of the active-matrix substrate is fabricated.
The opposite substrate (the color filer substrate) is fabricated in the following way. First, the color filter (not shown) and the light-shielding black matrix (not shown) are formed on one of the surfaces of the glass substrate 12 and thereafter, the overcoat film (not shown) is formed to cover the color filter and the black matrix over the whole surface of the glass substrate 12. Then, the columnar spacers (not shown) are formed on the overcoat film. In this way, the opposite substrate is fabricated.
The alignment films 31 and 32, which are made of polyimide, are formed on the surface of the active-matrix substrate and the surface of the opposite substrate fabricated as described above, respectively.
Next, the surfaces of the alignment films 31 and 32 are uniformly alignment-treated. These two substrates are then overlapped to have a constant gap (e.g., approximately 4.5 μm), and the peripheries of the coupled substrates are sealed by the sealing member except for an injection hole for the liquid crystal.
Next, in a vacuum chamber, a predetermined nematic liquid crystal (e.g., a nematic liquid crystal whose refractive index anisotropy is 0.067) is injected into the gap between the substrates through the injection hole and then, the injection hole is sealed.
After the substrates are coupled and unified in this way, the polarizer plates (not shown) are respectively adhered on the outer surfaces of the substrates. As a result, the related-art IPS-mode LCD device having the structure shown in FIG. 11A through FIG. 11C is completed.
With the related-art IPS-mode LCD device described above, it is known that the LC molecules are rotated to the direction opposite to the ordinary rotation direction in some regions (which are termed “reverse rotation domains”) when the LC driving electric field is applied.
FIG. 13 is a drawing schematically showing the generation principle of the reverse rotation domains in the related-art LCD device shown in FIGS. 11-12. To facilitate the explanation, only the pixel electrode 71, the common electrode 72 and the LC molecules 21 are shown in FIG. 13. In FIG. 13, the LC driving electric field 100 (its electric lines of force), which is generated by the comb-tooth-like portions of the pixel electrodes 71 and the common electrode 72, is schematically illustrated.
The rotation direction 27 of the LC-molecules 21 (the rotation of the LC-molecules 21 is caused in planes approximately parallel to the active-matrix substrate and the opposite substrate) is defined by the relationship between the initial alignment direction 30 of the LC-molecules 21 and the direction of the LC driving electric field.
Therefore, the rotation direction 27 of the LC-molecules 21 is “clockwise” in almost all the pixel region. However, in the vicinities of the comb-tooth-like portions of the pixel electrode 71, the LC driving electric field is radial, as shown in FIG. 13. Thus, the LC-molecules 21 are rotated “counterclockwise” in the shadowed regions in the drawing. This means that the shadowed regions are the reverse rotation domains 80 where the LC-molecules 21 are rotated “counterclockwise”.
Furthermore, Japanese Unexamined Patent Publication No. 10-307295 (patent document 5) published in 1998 discloses a technique that the electrodes for generating lateral electric field are bent to intentionally make the driving (rotating) direction of the LC molecules different in the respective regions with the bent parts of the electrodes, thereby reducing the display coloring in the slant views (see claims 1, 3 and 5, and FIGS. 1, 2, 4 and 6).
For example, the following structure is proposed. Specifically, the initial alignment direction of the LC molecules in the first sub-region is equalized to that in the second sub-region. When a voltage is applied, the LC molecules in the first and second sub-regions are rotated in opposite directions to each other while keeping the alignment directions of the LC molecules symmetrically in the first and second sub-regions (see claim 3). With this structure, preferably, the lateral electric field for driving the LC molecules is generated by the parallel electrode pair, and the electrodes constituting the parallel electrode pairs are bent into a V shape (see claim 5).
According to the conventional configuration shown in FIG. 13, in the vicinities of the comb tooth electrode tip portions, the LC driving electric field is distributed in a radiation shape, and being associated with an initial alignment direction of the liquid crystal, the area (the reverse rotation domains 80) are formed such that the LC molecules 21 rotate reversely opposite to the predetermined direction of its rotation. Because the LC driving electric field is gentle radiation shape in the vicinities of the comb tooth electrode tip portions, dark areas (i.e., boundary domains 90) which occurs between the reverse rotation domains 80 and the ordinary domains 70 becomes large. In addition to that, its position is unstable.
Accordingly, when the external pressure such as a finger pressing is added to the display surface, the state of the reverse rotation domains 80 (or, the position of the boundary domains 90) does not become stable, and after releasing the external pressure, it is recognized as the finger pressing scar. Because the width of each boundary domain 90 also becomes large, there is a problem that the loss of the optical transmission factor for the panel is produced. Although the reverse rotation domains 80 contribute to the optical transmission factor, the boundary domains 90 are still the dark condition at the time of the white display (i.e., at the time of applying voltage).